Although especially applicable to “The Input/Loss Method” as installed at recovery boilers burning black liquor, this invention may also be applied to any other of the “Input/Loss methods” installed at any thermal system burning a fossil fuel. The teachings of this invention may be implemented for monitoring of any thermal system burning a fossil-fuel, or a thermal system burning a mix of fossil fuels and inorganic fuels. Such monitoring is assumed to be conducted in a continuous manner (i.e., on-line), processing one monitoring cycle after another, each cycle includes determining stoichiometric balances of the combustion process and, specifically, the fuel's chemistry, heating value, boiler efficiency, system efficiency and evaluation for possible tube failures. Specifically, The Input/Loss Method and its associated technologies are described in the following U.S. Patents (cited above): U.S. Pat. No. 6,522,994 (hereinafter termed '994), U.S. Pat. No. 6,584,429 (hereinafter termed '429), U.S. Pat. No. 6,560,563 (hereinafter termed '563), U.S. Pat. No. 6,714,877 (hereinafter termed '879 after its application Ser. No. 10/087,879), U.S. Pat. No. 6,651,035 (hereinafter termed '035) and U.S. Pat. No. 6,745,152 (hereinafter termed '932 after its application Ser. No. 10/131,932). One of the Input/Loss methods, a rudimentary method, is described in U.S. Pat. No. 5,367,470 issued Nov. 22, 1994 (hereinafter termed '470), and in U.S. Pat. No. 5,790,420 issued Aug. 4, 1998 (hereinafter termed '420).
Conventional steam generators and recovery boilers having gas-to-working fluid heat exchangers, may be prone to tube leaks of their working fluid (typically water as liquid or steam). These tube leaks represent a potential for serious physical damage to heat exchangers due to pipe whip (i.e., mechanical movement) and/or steam cutting of metal given high leakages flowing at critical velocities. In some recovery boilers, pressures of the working fluid may exceed 2300 psia. Given failure of a heat exchanger tube, such fluid will experience many times critical pressure ratio as it expands into the combustion gases; that is, mixing with the products of combustion at essentially atmospheric pressure. When undetected, the damage from such tube failures may range from $2 to $10 million/leak forcing the system down for major repairs. If detected early, tube failures may be repaired before catastrophic damage, such repairs lasting only several days and costing a fraction of the cost associated with late detection and catastrophic damage. Repair times may be further reduced if the location of the heat exchanger which has the leak is identified before repairs are initiated.
However, an unique situation found with recovery boilers is associated with the pulp producing process to which they are integrated. This process involves first de-barking and chipping wood; then digesting the wood in an aqueous solution of NaOH and Na2SO4 (or other sodium-based compounds), forming a “white liquor”; then heating the brew; then separating the pulp from the spent liquor, the spent liquor is termed “black liquor” which consists of organics, water and inorganics (mostly sodium); and then the black liquor is burned in a recovery boiler. The essential function of a recovery boiler is the reduction in the furnace of sodium sulfate (Na2SO4, present in the black liquor) to sodium sulfide (Na2S). The efficiency of this sulfur reduction process is gauged by a “Reduction Efficiency” parameter. Heat from combustion of the fired organics, originating from the wood digesting process, generates steam. Black liquor inorganics, after reduction, are collected at the bottom of the furnace as a molten smelt, removed and recycled to recover sodium. Given a high Reduction Efficiency, smelt principally consists of Na2CO3, Na2S, inerts and free carbon.
The problem of tube failures in recovery boilers, in addition to the conventional problems cited above, is when water comes in contact with the molten smelt (typically at over 1400 F, having a heavy concentration of sodium); explosion is likely and may occur within minutes after tube failure. Recovery boiler explosions have dogged the pulp and paper industry since inception of the pulp producing process (i.e., called the Kraft process). Recovery boiler explosions injure and kill people every year. From 1948 through 1990 the industry recorded 140 recovery boiler explosions, three-quarters of which were smelt-water explosions. To place emphasis on the problem, the industry ranks explosions by severity: by definition just a “moderate explosion” keeps the plant off-line from 10 to 50 days; whereas a severe explosion keeps the plant off-line more than 50 days (typically lasting more than 120 days).
As common with conventional steam generators, tube failures in recovery boilers are typically caused by one the following general categories:                Weld failure of heat exchanger tubes;        Metallurgical damage caused by hydrogen absorption in the metal resulting in either embrittlement or the formation of non-protective magnetite;        Caustic gouging caused by the presence of free hydroxide in the water;        Corrosion-fatigue damage from the water-side of the tube, compounded by stress;        Corrosion damage caused by impacts from solid ash particles;        Fatigue failure caused by oxidation and/or mechanical movement, compounded by stress;        Overheating (e.g., from tube blockage) causing local creep; and        Physical damage from steam cutting and/or mechanical movement associated with another failed tube in the same locale.Commonly, the physical leak initiates as a relatively small penetration, although initial breaks may also occur. For reference and further discussion see: Chapter 18, “Failure Analysis and In-Service Experience—Fossil Boilers and Other Heat Transfer Surfaces” of The ASME Handbook on Water Technology for Thermal Power Systems, P. Cohen, Editor, The American Society of Mechanical Engineers, New York, N.Y., 1989; and J. Gommi, “Root Causes of Recovery Boiler Leaks”, 1997 Engineering and Papermakers Conference, TAPPI Proceedings, available from TAPPI Press as product code ENG97509, Atlanta, Ga.        
Present industrial art associated with conventional steam generators and recovery boilers have practiced the detection of tube failures using one or more of six general methods: 1) operator interface; 2) acoustic monitoring; 3) water balance testing; 4) monitoring of effluent moisture using instrumentation located at the system's effluent boundary (i.e., Stack); 5) monitoring the concentration of chemicals added to the working fluid whose change is sensitive to leakage; and 6) through use of artificial neural network technologies. Operator interface involves the use of his/her knowledge, experience, listening skills, and visual skills using remote cameras. However, all operators do not have the same high skill-set required. Acoustic devices detect the unique noise created by fluids at high velocities. However, acoustic devices rarely work in large steam generators, are expensive and require benchmarking with known acoustical signatures. Water balance testing may be conducted periodically on the entire system through which large water losses due to tube failures might be discovered. However, water balance testing is expensive, insensitive to small leaks, and typically may not be conducted at sufficient frequency to prevent serious damage. The use of an effluent moisture instrument has been shown to be sensitive to tube failures. Effluent moisture instrumentation may not differentiate between originating sources of water (e.g., between high humidity in the combustion air, or high fuel water, or changing fuel water, or a tube leakage). However, it might be practical to detect tube leakage by monitoring the difference in signals, or the rate of change of the difference in signals, between an effluent moisture instrument and one monitoring ambient air. Note that typical black liquor fuels contain up to 35% fuel water (approximately the same amount of water as found in some Powder River Basin (PRB) coals); thus changes in effluent moisture, even referenced to an ambient measurement, may be insensitive to small tube failures.
Monitoring the concentration of chemicals added to the working fluid operates by making a chemical mass balance on the working fluid based on a combination of flow measurements and chemical concentration measurements. Computed is a mass balance of a specific stable and non-volatile species (such as phosphate or molybdate) which has been uniquely added to the working fluid of the boiler. Basically a foreign chemical is injected into the working fluid; when a tube leak occurs the concentration of the chemical will change, thus detection. This method, developed by Burgmayer, Hong and Gunther, is described in U.S. Pat. No. 6,484,108 issued Nov. 19, 2002. A similar method is described in U.S. Pat. No. 3,522,008 issued Jul. 28, 1970. A similar method is also described in U.S. Pat. No. 5,320,967 issued Jun. 14, 1994. None of these methods involve combustion gases nor any stoichiometric balance involving the combustion process. The most serious limitation of these methods is their lack of sensitivity. Burgmayer's, Table 4 presents results of actual tests indicating that for a boiler producing 500,000 lb/hr of steam, detection of a 0.76% leakage (3,800 lb/hr) took 45 minutes, while a 2.8% leakage (14,000 lb/hr) took 15 min. to detect. Such sensitivities are not adequate to safeguard operators and equipment.
U.S. Pat. No. 6,192,352 by Alouani issued Feb. 20, 2001 (and his U.S. Patent Application Publication 2001/0001149 of May 10, 2001) discloses a method to detect tube failures using artificial neural network and fuzzy logic technology (ANN). No where in Alouani's patent is explicit thermodynamic modeling taught. Alouani's patent teaches that ANN technology may learn to predict tube failures through recognition of patterns in raw data. Such raw data may include coal pulverizer flow (fuel flow), boiler drum pressure, reheat temperature, burner tilt positions, etc. The disadvantage to this method is that it requires a database from which it may learn. Such a database, as must be associated with an actual tube leak, is not frequent, inconvenient and could not contain a defined tube leakage flow rate (explicit system mass balances are not mentioned nor inferred by Alouani). In addition, Alouani's patent FIG. 5 indicates that a number of days is required for his system to detect a tube leak. In a survey of critical tube leaks in recovery boilers, it was found that approximately half of those leaks for which the time between leak initiation and explosion was known, the explosion occurred within 15 minutes after leak initiation; approximately 75% of the recorded explosions occurred within 30 minutes. This survey's reference is: D. G. Bauer and W. B. A. Sharp, “The Inspection of Recovery Boilers to Detect Factors That Cause Critical Leaks”, TAPPI Journal, September 1991, TAPPI Press, Atlanta, Ga. Although there are more than a half-dozen vendors offering one or more of the six tube leak detection methods, in practice all known methods suffer serious short-comings and are not reliable in detecting early tube failures.
The patents '470 and '420 make no mention of heat exchanger tube failures nor their detection. Although the technologies of Patents '994, '429 and '563 generally support this invention, they make no mention of tube failure detection nor their location. Applications '879, '035 and '932 support this invention directly. Although the methods of '879, '035 and '932 are useful, the present invention further improves these methods and applies them to recovery boilers. There is no established art directly related to this invention; there is clear need for early detection of tube failures and to determine their location within the recovery boiler system.